hastens the ripening process; hence, this element
seems to act as the exact antithesis of nitrogen.

The role of phosphorus, or of phosphates, in the physiological processes of
the cell seems to be difficult to discover. The element itself is a
constituent of some protein complexes and of the lecithin-like bodies (see
page 141) which are supposed by some investigators to play an important
part in determining the rate of chemical changes which take place in the
cell and the movement of materials into and out of it. It is an essential
constituent of the nucleus, and a meager supply of phosphorus retards, or
inhibits, mitotic cell-division. Photosynthesis of sugars and the
condensing of these into starch or cellulose takes place in plants in the
absence of available phosphorus; but the change of these insoluble
carbohydrates back again into soluble and available sugar foods does not.

Phosphorus is taken from the soil by plants in the form of phosphates. Much
study has been given to the problem of the proper supply of available soil
phosphates for economic crop production. Any discussion of soil fertility
and fertilization which did not devote large attention to the conditions
under which phosphates become available as plant food would be wholly
inadequate; but such a discussion would be out of place here.

The final result of an ample supply of phosphates in hastening the ripening
process and stimulating seed production, as contrasted with that of an
over-supply of nitrogen, has led to the popular statement that "phosphates
make seeds." This statement, while not strictly accurate, is a fairly good
summary of the combined results of the role of phosphorus in the plant
economy. Large amounts of phosphorus are stored in the seeds. The two facts
that large amounts of these compounds are thus available to the young
seedling and that relatively large proportions of phosphates are taken from
the soil by the plant during its early stages of growth are undoubtedly
connected with the need for rapid cell-division at these periods in the
plant's life.

=Potassium.=--The popular expression that "potash makes sugars and starch"
is a surprisingly accurate description of the role of this element in plant
metabolism. Either the photosynthesis of starch, or the changes necessary
to its translocation (it is not yet certain which) is so dependent upon the
presence of potassium in the cell sap that the whole process stops at once
if an ins